The presently disclosed embodiments relate generally to a novel overcoat layer formulation based on cross-linkable polyester resins that is used to form a cross-linked protective outer coating or layer on a photoreceptor. The overcoat layer not only provides wear resistance, but it also provides higher charge transport efficiency and therefore better photoelectrical properties. In addition, the polyesters can cross-link with a variety of resins and thus provide good adhesion as well.
In electrophotographic or electrostatographic printing, the charge retentive surface, typically known as a photoreceptor, is electrostatically charged, and then exposed to a light pattern of an original image to selectively discharge the surface in accordance therewith. The resulting pattern of charged and discharged areas on the photoreceptor form an electrostatic charge pattern, known as a latent image, conforming to the original image. The latent image is developed by contacting it with a finely divided electrostatically attractable powder known as toner. Toner is held on the image areas by the electrostatic charge on the photoreceptor surface. Thus, a toner image is produced in conformity with a light image of the original being reproduced or printed. The toner image may then be transferred to a substrate or support member (e.g., paper) directly or through the use of an intermediate transfer member, and the image affixed thereto to form a permanent record of the image to be reproduced or printed. Subsequent to development, excess toner left on the charge retentive surface is cleaned from the surface. The process is useful for light lens copying from an original or printing electronically generated or stored originals such as with a raster output scanner (ROS), where a charged surface may be imagewise discharged in a variety of ways.
The described electrostatographic copying process is well known and is commonly used for light lens copying of an original document. Analogous processes also exist in other electrostatographic printing applications such as, for example, digital laser printing or ionographic printing and reproduction where charge is deposited on a charge retentive surface in response to electronically generated or stored images.
To charge the surface of a photoreceptor, a contact type charging device has been used. The contact type charging device includes a conductive member which is supplied a voltage from a power source with a D.C. voltage superimposed with a A.C. voltage of no less than twice the level of the D.C. voltage. The charging device contacts the image bearing member (photoreceptor) surface, which is a member to be charged. The outer surface of the image bearing member is charged with the rubbing friction at the contact area. The contact type charging device charges the image bearing member to a predetermined potential. Typically the contact type charger is in the form of a roll charger such as that disclosed in U.S. Pat. No. 4,387,980, the relative portions thereof incorporated herein by reference.
Multilayered photoreceptors or imaging members have at least two layers, and may include a substrate, a conductive layer, an optional undercoat layer (sometimes referred to as a “charge blocking layer” or “hole blocking layer”), an optional adhesive layer, a photogenerating layer (sometimes referred to as a “charge generation layer,” “charge generating layer,” or “charge generator layer”), a charge transport layer, and an optional overcoating layer in either a flexible belt form or a rigid drum configuration. In the multilayer configuration, the active layers of the photoreceptor are the charge generation layer (CGL) and the charge transport layer (CTL). Enhancement of charge transport across these layers provides better photoreceptor performance. Multilayered flexible photoreceptor members may include an anti-curl layer on the backside of the substrate, opposite to the side of the electrically active layers, to render the desired photoreceptor flatness.
Extending the lifetime of xerographic imaging members creates challenges in meeting the critical quality requirements, in particular for bias charge roll-based engines, where the contact charging is notorious for causing abrasion and related or unrelated print defects. To improve robustness against mechanical wear, there are two commonly-used methods—one is to enhance wear resistance of charge transport layer and the other is to apply a protective overcoat. Each method has its own advantages and disadvantages, however, it is predicted that life extension in the future will be based on some form of overcoat layer. One serious concern with using overcoat layers is the compromise on electrical performance, namely, the photoinduced discharge characteristics (PIDC) curve becomes “softer”, i.e. increases of surface potential, with the presence of an overcoat layer, making many overcoat layers not suitable for xerographic applications.
Therefore, a need remains for a photoreceptor overcoat layer that can provide wear resistance without adversely impacting electrical performance of the photoreceptor.